Allan NantelRoom MA202

FADE South BuildingOctober 2011

CONCRETE IN CONSTRUCTION

Faculty of Applied Design and EngineeringSchool of The Built and Natural

Environment

How old is the use of concrete in construction?

Concrete

http://youtu.be/5IOD72Kdx3A

The Romans first invented what today we call a hydraulic cement-based concrete. They built numerous concrete structures, including the Pantheon in Rome, one of the finest examples of Roman architecture that survives to this day (1st century BC)

The word ‘concrete’ comes from the Latin word "concretus" (meaning compact or condensed),

History of Concrete

Rome - The PantheonThe largest non-reinforced solid concrete dome , 42-meter-diameter, made of poured concrete.

Wall at Palestrina, Italy, 1st Century BC

1756, British engineer, John Smeaton manufactured the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into a lime cement.

1824 an English inventor Joseph Aspdin invented ‘Portland’ cement from a heated mixture of limestone and clay. Named after colour of Portland’s Jurassic limestone, South Dorset.

In 1849, Joseph Monier invented reinforced concrete

History of Concrete

Concrete types & properties

Concrete is a composite construction material, composed of: Cement – The binding agent

Coarse Aggregate – gravel, crushed rock.

Fine aggregate – sands

Water

Chemical admixtures

What is Concrete ?

Concrete Types Plain Mass - no reinforcement, dams, retaining wall.

Lean – large ratio of aggregate, for fill not structural.

Structural – load bearing, high density.

Reinforced – steel assisted strength.

Pre-stressed – subjected to pre-placement compression to counteract expected tensile forces.

Pre-cast – moulded, channels, culverts.

Water resistant – higher density with spray lacquer, asphalt coated- used for water tanks.

High density – utilises haematite, iron or steel shot as aggregate. Used for shield walls, sea walls.

Fibre reinforced – uses nylon, steel, carbon fibres as reinforcement. POLYSTYRENE !!

High Alumina – high early strength, resistance to high temperatures and sulphates. Copy Down

Hardening of concrete Starts by gradual stiffening (setting). Progressive increase in strength

(hardening). Hardening is by chemical action –

(hydration).

Exothermic reaction – the quantity of heat produced by a unit mass of cement is known as it’s Heat of Hydration

Factors affecting that Setting/Hardening Composition – Relative oxide combinations.

Temperature – Increased temp. gives increased rate.

Moisture – For ultimate strength, it must be kept moist (Cured)

Additives –

Gypsum is a retardent and is added during manufacture to slow the setting rate.

Calcium Chloride is an accelerator, increasing the rate, used in winter conditions.

(Not used in RC’s as there is a risk of corrosion of steel)

Is the relative weight of water /cement in the mix. This ratio controls the properties of the mix by

giving control over strength and workability. Higher water content will give workability but

reduce strength Lower water content increases strength but

lowers workability. We design the mix for each specified requirement.

Potable water is most suitable for use…..sea water can be used….but will affect reinforcement.

Concrete – The W/C ratio

1. Cement

A cement is a binder, a substance that sets and hardens independently, and can bind other materials together.

Cement used in construction is characterized as hydraulic or non-hydraulic.

Hydraulic cements (e.g., Portland cement) harden because of hydration, chemical reactions that occur independently of the mixture's water content; they can harden even underwater or when constantly exposed to wet weather.

The chemical reaction that results when the anhydrous cement powder is mixed with water produces hydrates that are not water-soluble.

Non-hydraulic cements (e.g., lime and gypsum plaster) must be kept dry in order to retain their strength.

Cement

BS EN197-1:2000 lists 5 main types of cement :-

◦ CEM I Portland cement◦ CEM IIPortland composite cement◦ CEM III Blastfurnace cement◦ CEM IV Pozzolanic cement◦ CEM V Composite cement

The standard strength classes of cement are based on the 28 day compressive strength of mortar prisms (BS EN196-1:2005)

Classification of Cement

Each cement strength class (32.5, 42.5 and 52.5) has sub classes associated with the following :-

◦ Ordinary early strength development (N)◦ High early strength development (R)

Strength classes and sub classes give production standards for cement, but do not specify how a particular mix of cement, aggregate and admixtures will perform as a concrete; this needs to be determined by separate testing. We will look at these tests later.

Strength classes of Cement

Most commonly used cement (OPC) in UK :-

◦ CEM I 42.5 N CEM I 42.5N type of cement strength class ordinary early

strength dev

High early strength Portland cement

◦ CEM I 42.5 R CEM I 42.5R type of cement strength class high early

strength dev

Strength classes of Cement

Portland Cement

Portland cement (often referred to as OPC, from Ordinary Portland Cement) is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar and most non-specialty grout.

It is a fine powder produced by grinding Portland cement clinker (more than 90%), a limited amount of calcium sulphate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as the European Standard EN197-1

Portland Cement

Limestone / chalk is a naturally occurring mineral that consists principally of calcium carbonate (CaCO3).

Lime is manufactured by calcining natural calcium carbonate, typically hard rock carboniferous limestone.

CaCO3 + heat CaO + CO2

calcium carbonate lime carbon dioxide

The whole process of making any type of lime all begins back at the limestone quarries.

Manufacture of Lime

At the quarry, careful surveys and preparation is carried out into locating and drilling holes behind the rock face into which explosives are placed.

When detonated, the explosion dislodges up to 30,000 tonnes of stone each time.

This is then picked up at the quarry ‘face’ by huge, mechanised excavators which work along a bench of rock.

Limestone quarry

Limestone quarry

Limestone quarryTypically these ‘benches’ have rock ‘faces’ about 20 metres high. The excavators then either load the stone into equally large tipper trucks, each carrying up to 100 tonnes of stone per trip or on to a conveyor system.

The limestone / chalk is transported across the quarry to begin its’ processing.

Crushing limestone A large primary crusher which impacts or compresses the rock.

Depending on the size of the feedstone required and the kiln in which it will fed into, the same stone can go through a second and even tertiary crusher to reduce its mass even further.

The stone is then screened into a wide range of different sizes from 125mm kiln stone all the way down to dust. Some of the stone at this point is washed to remove any clay particles that may remain.

Portland cement• Raw materials

• Limestone (Calcium Carbonate)

• Silica• Alumina (in clay or

shale)• Ironstone

Is South Wales well placed to produce Cement ??

Portland cement Aggregate ‘golf ball’ size passes through rotating kiln. Kiln is

angled and rotates slowly, pushing aggregate from one end to another and gradually heating it to 1900 degrees. Carbon Dioxide gas is burnt off. Limestone decomposes to Calcium Oxide (quick lime)

Reacts with additives (silica, alumina, iron) to form calcium aluminates , silicates and alumino-ferrites.

Dry state mix of these is called ‘clinker’.

Red hot clinker is discharged into coolers and then stock piled.

This is then ground to the required fineness to form cement, with the addition of gypsum (Calcium Sulphate) usually 4-7% by weight, which controls the setting time.

Portland cement manufacturing

◦ Tri-calcium silicate 3CaO.SiO2 = C3S

◦ Di-calcium silicate 2CaO.SiO2 = C2S

◦ Tri-calcium aluminate 3CaO.Al2O3 = C3A

◦ Tetra-calcium aluminoferrite 4CaO.Al2O3.Fe2O3 = C4AF

C3S and C2S ◦ are responsible for the strength of hydrated cement paste

C3A is present in small quantities. Reacts very rapidly with water (‘flash set’) (The addition of gypsum retards this).◦ provides early strength◦ prone to sulphate attack

C4AF ◦ dark in color with little cementing value◦ iron oxide, a useful flux during the burning process.

Main Components of Portland Cement

Properties of the major constituents of cement

Characteristics Heat ofHydration(J/g)

C3S Light in colorHardens quicklyGives early strength

500

C2S Light in colorHardens slowlyGives late strength

250

C3A Light in colorSets quicklyEnhances strength of silicates

850

C4AF Dark in colorLittle cementing value

400

Provides Flux

White Portland cement◦ Manufactured from materials free of iron oxide and impurities

which impart the grey colour to Portland cement◦ Usually twice the price of normal Portland cement because of

specialist manufacturing process◦ White titanium oxide may be added to enhance the white colour◦ Used for renderings, cast stone, pre cast and in situ structural

concrete

Sulphate resisting Portland cement◦ Suitable for concrete and mortar in contact with soils and

groundwater containing soluble sulphates for better durability◦ Also low in alkali

Additional Types of Portland Cement

Very low heat special cements

◦ Appropriate for use in mass concrete , where rapid internal evolution of heat could cause cracking

◦ Suitable for dam construction, but not for bridges and buildings

Types of Portland Cement

Blended Portland cement includes ◦ Masonry cement

Used for mortar Accommodate differential movement Contains water retaining mineral fillers & air entraining agents

to give high workability

◦ Blast furnace cement Material is a by product of the iron making process of the steel industry Slag is ground to a fine white powder added to the clinker in the cement

mill Concrete manufactured from the blend has lower permeability than OPC

alone This enhances resistance to attack from sulphates and weak

acids which can cause corrosion on reinforcement

Types of Blended Portland Cement

Natural pozzolans are volcanic in origin.

In UK, pulverised fuel ash (PFA), the waste product from coal fired electricity generating station is used as a blend with Portland cement to make Pozzolanic cement.

PFA cement cures and evolves heat more slowly than Portland cement. It is therefore appropriate for use in mass concrete to reduce the risk of thermal cracking

Has good sulphate resisting properties & resistance to chlorides

Pozzolanic cements are used for large hydraulic structures such as bridge piers and dams

Types of Pozzolanic cement

2. Aggregates

Aggregates are inert granular materials such

as sand, gravel, or crushed stone that, along

with water and Portland cement, are an

essential ingredient in concrete. For a good concrete mix, aggregates need to

be 1. Clean 2. Hard 3. Free of organic impurity : absorbed chemicals (sulphates) or coatings

of clay and other fine materials that could

cause the deterioration of concrete.

Aggregates

As aggregates account for 60 to 75% of the total volume of concrete, the selection of suitable material is important as it can affect its performance.

Sources of aggregates ◦ Naturally occuring aggregates

Rocks reduced to a suitable size by natural process of weathering or by mechanical crushing

Obtained from river beds, quarries, sea beds and cooled volcanoes◦ Artificial (man made) aggregates

Obtained as waste products from kilns, blast furnaces etc◦ Recycled aggregates

Resulting from the processing of inorganic materials e.g. construction and demolition waste

Sources of Aggregates

Functions of Aggregates

Aggregates that are used to make concrete have the following important functions :-

◦ Reduce costs - by bulking the mix◦ Modify properties of concrete by

Increase or decrease density Increase durability Increase fire resistance Increase sound and heat insulation Change colour / texture Improve workability

Functions of Aggregates

Large aggregates:◦ provide density (fill space)◦ provide strength

Fine aggregates:◦ fill small voids between large

aggregates◦ Increases strength of the cement

binder

Types of Aggregates 3 types of aggregates

◦ Normal• Mostly used to produce general purpose dense

concrete• Density of 1400-1800kg/m3

◦ Heavy• Used to produce high density concrete (eg in nuclear power station, transformer housings etc)

◦ Lightweight• Used to produce lightweight concrete which has good heat & sound insulation• Porous and density of 50 – 1200 kg/m3

Types of AggregatesTypes of Aggregates

Natural Man made

1) Normal Density • Gravel • Made up of small naturally

worn stones• Round in shape• Originated from quartz rock

• Sand • Smaller fragments of

quartz (silica) than gravel• Formed by natural

disintegration of quartz rock• Crushed stone

• Limestone, granite and sandstone are quarried and crushed to the size needed for use as aggregates

• Blast furnace slag• Waste product

from production of pig iron

• Molten slag is cooled and crushed

• Crushed brick• Waste bricks are

cleaned and mechanically crushed to a suitable size

• Bricks should have low sulphate content

Beach sands are generally unsuitable for quality mixes because of salt content, shell fragments and they are often single sized.

Types of AggregatesTypes of Aggregates

Natural Man made

2) Heavy • Iron ores• Haematite• Magnetite

• Barium containing ores (barytes)

• Small globules of steel

Types of AggregatesTypes of Aggregates

Natural Man made

3) Lightweight

• Expanded clay or shale• When heated to melting

point, they expand and air is trapped in their interior, forming cellular structure

• Expanded perlite• Glassy volcanic rock• When heated quickly, it

expands to give porous cellular structure

• Pumice• Ideal porous lightweight

aggregate of volcanic origin

• Clinker• Residue from furnaces• Harmful impurities must be

removed as they may attack cement or reduce binding power of cement

• Foamed blast furnace slag• Molten slag is cooled by

blowing air or steam through it resulting in air bubbles formation and hence cellular structure

• Sintered pulverised fuel ash• Ash from coal burnt at power

stations is mixed with water and heated (sintered) , resulting in cellular pellets which has air pockets

Particle Shapes & Texture Particle shape & texture affects the workability of

concrete mix and binding strength of cement

◦ Smooth rounded particles give maximum workability

◦ Rough, cuboidal particles give optimum strength

◦ Flaky, elongated particles requires more water and are prone to segregation and high stress concentration, hence reduction in strength.

Particle Shapes & Texture The rougher the surface texture, the

greater the binding strength between aggregate and cement

6 types of surface texture◦ Glassy

◦ Conchoidal ( i.e. curved) fracture◦ Smooth

◦ Water worn or smooth due to fracture of laminated or very finely grained rock

◦ Granular◦ Fracture showing more or less uniform

size rounded grains

Particle Shapes & Texture 6 types of surface texture (cont’d)

◦Rough◦ Fracture of fine or medium grained rock

containing no easily visible crystalline constituents

◦Crystalline◦ Containing easily visible crystalline constituents

◦Honeycombed◦ With visible pores and cavities

Size of Aggregates As a general rule, the maximum size of

aggregate should be as large as possible since this reduces the quantity of sand and therefore cement required in the mix, thus controlling shrinkage and minimising cost.

However, there are constraints in maximising aggregate size as workability and strength should not be compromised.

Size of Aggregates The maximum coarse aggregate sizes with typical

applications are:-

40 mmMass concrete, road construction 20 mmGeneral concrete work, including reinforced & prestressed concrete 10 mmThin sections, screeds > 50 mm thickness 5 mmScreeds ≤ 50 mm thickness

A coarse aggregate should have a particle size > 4mm, and a fine aggregate < 4 mm.

All-in aggregates are natural mixes of coarse and fine articles

Grading of Aggregates

The proportions of the different sizes of particles making up the aggregate are found by sieving and are known as “grading” of the aggregate.

Grading of Aggregates The grading determines the

paste requirement for a workable concrete since the amount of void required needs to be filled by the same amount of cement paste in a concrete mixture.

To obtain a grading curve for aggregate, sieve analysis has to be conducted.

Grading of Aggregates1. The sample is dried in a kiln, cooled and weighed.

2. The sample is then passed through a series of sieves of reducing mesh size by mechanical vibration.

3. When this is complete, the amount retained on each sieve is weighed and expressed as a percentage of the original sample weight.

4. The cumulative percentage by weight of the sample % passing each sieve is recorded graphically.

Grading of Aggregates 5 different kinds of size distributions:

dense graded well graded

gap graded

Desirable for making concrete, as the space between larger particles is effectively filled by smaller particles to produce a well-packed structure.

• Lacks one or more intermediate size.• Can make good concrete when the required workability is relatively low. • Segregation may become a problem in high workability mixes.

Grading of Aggregates 5 different kinds of size distributions (cont’d):

uniform graded

open graded

A wide range of grading curves is acceptable for the economic production of concrete with good quality. As long as the grading curve lies within the recommended grading limits, the aggregate can be employed.

• Only a few sizes dominate the bulk material. • Not effectively packed, and the resulting concrete will be more porous, unless a lot of paste is employed.

Contains too much small particles and easy to be disturbed by a hole.

Grading of Aggregates

Properties of Aggregates Moisture condition of Aggregates

Refers to the presence of water in the pores and on the surface of aggregates.

4 different moisture conditions: Oven Dry (OD): This condition is obtained by keeping aggregates at temperature of 1100C for a period of time long enough to reach a constant weight

Air Dry (AD): This condition is obtained by keeping aggregates under room temperature and humidity. Pores inside the aggregate are partly filled with water.

•

Properties of Aggregates4 different moisture conditions: Saturated Surface Dry (SSD):

In this situation the pores of the aggregate are fully filled with water and the surface is dry. This condition can be obtained by immersion in water for 24 hours following by drying of the surface with wet cloth.

Wet (W): The pores of the aggregate are fully filled with water and

the surface of aggregate is covered with a film of water.

•

Properties of Aggregates

Why does the aggregate moisture condition matter?

Density of Aggregates Density (D): weight per unit volume (volume excluding

the pores inside a single aggregate) D = Weight

Vsolid

Bulk density (BD) : weight per unit volume (volume includes the pores inside a single aggregate)

BD = Weight Vsolid + Vpores

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Excercise 2:

1. Write a paragraph on the types of aggregates available for use and the method for their grading.

2. Write a paragraph on how you think the surface area, shape and texture of an aggregate affect;a). The workability of the mixb). The strength of the hardened concrete.